The transforming growth factor-beta (TGF-Beta) (“TGF-β”) polypeptides influence growth, differentiation, and gene expression in many cell types. The first polypeptide of this family that was characterized, TGF-β1, has two identical 112 amino acid subunits that are covalently linked. TGF-β1 is a highly conserved protein with only a single amino acid difference distinguishing humans from mice. There are two other members of the TGF-β gene family that are expressed in mammals. TGF-β2 is 71% homologous to TGF-β1 (de Martin, et al., (1987) EMBO J 6:3673-3677), whereas TGF-β3 is 80% homologous to TGF-β1 (Derynck, et al. (1988) EMBO J. 7:3737-3743). The structural characteristics of TGF-β1 as determined by nuclear magnetic resonance (Archer, et al. (1993) Biochemistry 32:1164-1171) agree with the crystal structure of TGF-β2 (Daopin, et al. (1992) Science 257:369-374; Schlunegger and Grutter (1992) Nature 358:430-434).
There are at least three different extracellular TGF-β receptors, Type I, II and III that are involved in the biological functions of TGF-β1, -β2 and -β3 (For reviews, see Derynck (1994) TIBS 19:548-553 and Massague (1990) Ann. Rev. Cell Biol. 6:597-641). The Type I and Type II receptors are transmembrane serine/threonine kinases which in the presence of TGF-β form a heteromeric signaling complex (Wrana, et al (1992) Cell 71: 1003-1014).
The mechanism of activation of the heteromeric signaling complex at the cell surface has been elucidated (Wrana, et al. (1994) Nature 370: 341-347). TGF-β first binds the type II receptor that is a constitutively active transmembrane serine/threonine kinase. The type I receptor is subsequently recruited into the complex, phoshorylated at the GS domain and activated to phosphorylate downstream signaling components (e.g. Smad proteins) to initiate the intracellular signaling cascade. A constitutively active type I receptor (T204D mutant) has been shown to effectively transduce TGF-β responses, thus bypassing the requirement for TGF-β and the type II receptor (Wieser, et al. (1995) EMBO J 14: 2199-2208). Although no signaling function has been discovered for the type III receptor, it does increase TGF-β2's affinity for the type II receptor making it essentially equipotent with TGF-β1 and TGF-β3 (Lopez-Casillas, et al. (1993) Cell 73: 1435-1444).
Vascular endothelial cells lack the Type III receptor. Instead endothelial cells express a structurally related protein called endoglin (Cheifetz, et al., (1992) J. Biol. Chem. 267:19027-19030), which only binds TGF-β1 and TGF-β3 with high affinity. Thus, the relative potency of the TGF-β's reflects the type of receptors expressed in a cell and organ system. In addition to the regulation of the components in the multi-factorial signaling pathway, the distribution of the synthesis of TGF-β polypeptides also affects physiological function. The distribution of TGF-β2 and TGF-β3 is more limited (Derynck, et al. (1988) EMBO J. 7:3737-3743) than TGF-β1, e.g., TGF-β3 is limited to tissues of mesenchymal origin, whereas TGF-β1 is present in both tissues of mesenchymal and epithelial origin.
TGF-β1 is a multifunctional cytokine critical for tissue repair. High concentrations of TGF-β1 are delivered to the site of injury by platelet granules (Assoian and Sporn (1986) J. Cell Biol. 102:1217-1223). TGF-β1 initiates a series of events that promote healing including chemo taxis of cells such as leukocytes, monocytes and fibroblasts, and regulation of growth factors and cytokines involved in angiogenesis, cell division associated with tissue repair and inflammatory responses. TGF-β1 also stimulates the synthesis of extracellular matrix components (Roberts, et al. (1986) Proc. Natl. Acad. Sci. USA 83:4167-4171; Sporn, et al. (1983) Science 219:1329-1330; Massague (1987) Cell 49:437-438) and most importantly for understanding the pathophysiology of TGF-β1, TGF-β1 autoregulates its own synthesis (Kim, et al. (1989) J. Biol. Chem. 264:7041-7045).